The use of outdoor playfields is more or less limited by the prevailing natural conditions. In the northern latitudes, acceptable winter use is not feasible, but in Central Europe, football fields are kept open through the winter by heating the field and removing snow which has accumulated on the surface of the field. In spring too, field use is limited by rainwater. Often, a good field drain network is arranged to remove water rapidly from the field. The surface of a field may also freeze in winter, in which case de-freezing is also necessary. A field that has not been used all winter can freeze to a considerable depth. In Northern America, similar problems occur with baseball and football fields.
Various systems have been developed for the de-freezing and drying of a playfield. These are electrical, fluid, and air heated systems. Electrical heating is implemented by means of electrical resistance elements buried in the field, fluid heating by communicating heated fluid through a network of heating pipes and air heating by communicating heated air through an air distribution pipe network.
Watering a playfield from below, as so-called damming watering, requires aerial irrigation even in connection with air blowing systems.
If a pipe network is used as a field drain system, one end of it must be located higher up than the rest of the system and form a continuously descending pipe structure for run-off water.
All in all, the most important tasks for the pipe network are the drying and de-freezing of the field with the aid of blow air and the removal of water from the field. The optimal range of moisture in the growing layer is typically 9-13%. This range depends on the type of ground among other things. The organic activities of plants demand a temperature of at least 7.degree. C.
Pipe networks suitable for blowing hot air in a field structure are shown in the German Patent Publications DE 924 931 and DE 938 850 as well as in German Patent Applications DE 2 059 383, 2 005 378, 2 005 412, and 2,738 133. The most common way of arranging the pipe network is shown in the first publication, in which the main channel is led longitudinally over the field and is connected to transverse multi-purpose pipes. Here these are used both to warm the field and to act as field drain pipes. Arranged in this way the distribution of the heat in the multi-purpose pipes is extremely uneven, because the pipes become half the length of the field. Otherwise too it is quite possible that the air flow in different parts of the field are not even, because air tries to escape through the point of least resistance.
Application publications 2,738,133 and 2,059,383 show pipe network systems that are divided into several parts, and in which the pipe network is fed from four separate blowers at the four corners. In this case, the distribution of heat and airflow is more even, because random variations between different parts of the field do not have as great of an effect as they would if the entire field had a single unified pipe network. In any event, in these too, the multi-purpose pipes for each section of the field become quite long. This pipe length can result in unevenness in the distribution of heat and airflow over the length of the pipe. Also, the use of four blowers mean large investments.
In order to improve the removal of water it is also possible to connect a pump or a vacuum source to the pipe network, as is shown for example in publications 2,738,133 and 2,005,378. This leads to a quite complicated arrangement, as can be seen in the latter publication.
The normal method of de-freezing a field is to remove any possible snow layer, and field heat it until the frozen water has melted and the field is dried. If there is much snow on the surface and the field is frozen to a substantial depth, the known systems require a great heat output.